1. Field of the Invention
The present invention relates to a switching power source apparatus, and particularly, to a technique of shortening a starting time of a start circuit of the switching power source apparatus and minimizing losses in the start circuit.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a switching power source apparatus according to a related art. In FIG. 1, the switching power source apparatus includes a DC power source E, a capacitor C12, a start circuit 1, a transformer T with a primary winding P1, secondary winding S, and tertiary winding P2, a switching element Q10 (e.g., a MOSFET), a resistor R10 for detecting a current passing through the switching element Q10, a control circuit 3 for controlling ON/OFF operation of the switching element Q10, a first rectify-smooth circuit having a diode D11 and capacitor C11, a second rectify-smooth circuit having a diode D10 and capacitor C10, and a detector 7.
The capacitor C12 represents an equivalent capacitor that is present at an input part of the switching power source apparatus, such as a smoothing capacitor to rectify and smooth AC power for the switching power source apparatus. Due to this configuration, an input voltage to the switching power source apparatus is not immediately zeroed when the DC power source E is cut. The start circuit 1 is connected between a positive terminal of the capacitor C12 and a power source input terminal of the control circuit 3 and is also connected to a first end of the primary winding P1 of the transformer T. The DC power source E always or intermittently applies power to the start circuit 1. The control circuit 3 becomes operative in response to a turn-on voltage Von (e.g., 18 V) and changes to inoperative in response to a turn-off voltage Voff (e.g., 9 V). Based on an output voltage Vout detected by the detector 7, the control circuit 3 turns on/off the switching element Q10, to maintain the output voltage at a predetermined voltage.
The start circuit 1 includes a series circuit connected between the first end of the primary winding P1 of the transformer T and a first end of the control circuit 3, the series circuit including a resistor R1, constant current circuit CC1, switch SW1, and diode D1. The start circuit 1 also includes a comparator CP. The comparator CP has an inverting input terminal connected to a connection point between a cathode of the diode D1 and the first end of the control circuit 3, anon-inverting input terminal connected to a reference power source Vr1, and an output terminal connected to a contact of the switch SW1. The comparator CP has a hysteresis characteristic so that it provides a low-level output when the inverting input terminal reaches, for example, 18 V and a high-level output when, with the output of the comparator CP being low, the inverting input terminal drops lower than, for example, 9 V.
Operation of the switching power source apparatus of FIG. 1 will be explained. When the DC power source E is enabled for the switching power source apparatus, a voltage Vst is applied through the resistor R1 to the constant current circuit CC1 in the start circuit 1. At this time, the switch SW1 is ON, and therefore, the constant current circuit CC1 passes a constant current Ist (e.g., 2.5 mA) to charge the capacitor C10 through the diode D1. A voltage of the capacitor C10 is supplied to the power source terminal of the control circuit 3. Namely, the control circuit 3 receives a voltage Vcc.
At the starting, the voltage Vcc to the control circuit 3 is lower than the turn-on voltage Von of 18 V, and therefore, the comparator CP provides a high-level output to maintain the ON state of the switch SW1. When the voltage Vcc reaches the turn-on voltage Von, the control circuit 3 starts to provide a drive signal Drv to turn on/off the switching element Q10. As a result, the primary winding P1 of the transformer T intermittently receives the DC power source E to induce a voltage on the secondary winding S. The voltage on the secondary winding S is rectified and smoothed with the diode D11 and capacitor C11 into an output voltage Vout that is applied to a load 5. The output voltage Vout supplied to the load 5 is compared with a reference voltage in the detector 7, which provides an error signal to the control circuit 3. The control circuit 3 generates the drive signal Drv whose duty factor is determined due to the error signal, to turn on/off the switching element Q10.
At the time when the voltage Vcc to the control circuit 3 reaches the turn-on voltage Von, the output of the comparator CP changes from high to low to turn off the switch SW1, to stop charging the capacitor C10. Irrespective of this, the tertiary winding P2 of the transformer T generates a voltage, which is rectified and smoothed through the diode D10 and capacitor C10 into a DC voltage. This DC voltage is supplied as Vcc to the control circuit 3 so that the control circuit 3 continuously operates. In this way, the starting current Ist is stopped once the control circuit 3 has started, to thereby improve efficiency.
FIG. 2 is a timing chart showing signals in the switching power source apparatus of FIG. 1 in a case where the DC power source E is cut and resumed. In FIG. 2, the DC power source E is enabled to start the switching power source apparatus, is once cut, and then, is again enabled.
At time of t1, the DC power source E is applied to the switching power source apparatus. Namely, the voltage Vst of the DC power source E to the resistor R1 starts to increase. At t2, the voltage Vst reaches a level to drive the constant current circuit CC1. The constant current circuit CC1 supplies the constant current Ist to charge the capacitor C10 and the voltage Vcc to the control circuit 3 is increased. At t3, the voltage Vcc reaches the turn-on voltage Von, and therefore, the control circuit 3 provides the drive signal Drv to turn on/off the switching element Q10. At the same time, the comparator CP of the start circuit 1 provides a low-level output to turn off the switch SW1.
At t4, the DC power source E is cut and the voltage Vst starts to decrease. At t5, the control circuit 3 becomes unable to control the output voltage Vout, and therefore, the output voltage Vout and the voltage Vcc to the control circuit 3 start to decrease. At t6, the voltage Vcc reaches the turn-off voltage Voff. Then, the comparator CP provides a high-level output to turn on the switch SW1, so that the starting current Ist supplied by the constant current circuit CC1 may charge the capacitor C10. If the DC power source E is continuously cut, the voltage Vst to the start circuit 1 further drops and the constant current circuit CC1 becomes unable to supply the starting current Ist. Then, the voltage Vcc to the control circuit 3 is unable to rise to the turn-on voltage Von and the switching power source apparatus becomes inoperative.
At t6 as shown in FIG. 2, the constant current circuit CC1 starts to supply the starting current Ist. At t7 during the period in which the starting current Ist is being passed, the DC power source E is restarted. The voltage Vst to the start circuit 1 starts to increase and the constant current circuit CC1 continuously supplies the starting current Ist to charge the capacitor C10. At t8, the voltage Vcc to the control circuit 3 reaches the turn-on voltage Von and the control circuit 3 provides the drive signal Drv to turn on/off the switching element Q10.
FIG. 3 is a timing chart showing signals in the switching power source apparatus of FIG. 1 when the switching power source apparatus conducts an auto-restart operation in an overload state. The auto-restart operation takes place when the load 5 encounters an overload state or a short-circuit state that stops the switching power source apparatus, to try to resume a normal operation of the switching power source apparatus once the overload state or short-circuit state resolves.
If an overload state occurs, a current passing through the switching element Q10 increases and a voltage at the current detection resistor R10 increases. This voltage is detected at t1 by the control circuit 3. If the overload state continues for a predetermined delay time after detecting the voltage, the control circuit 3 stops at t2 the drive signal Drv to the switching element Q10. As a result, the output voltage Vout and the voltage Vcc to the control circuit 3 decrease, and at t3, the voltage Vcc reaches the turn-off voltage Voff. This results in turning on the switch SW1 and charging the capacitor C10 with the current Ist from the constant current circuit CC1, to increase the voltage Vcc. At t4, the voltage Vcc reaches the turn-on voltage Von, so that the control circuit 3 resumes the drive signal Drv.
If the overload state continues for the delay time, the control circuit 3 again stops at t5 the drive signal Drv to the switching element Q10. These actions are repeated until the overload state dissolves. When the overload state is cleared, a normal operation resumes. During the overload state, the start circuit 1 supplies the starting current Ist intermittently so that the switching element Q10 intermittently conducts ON/OFF operation under the overload state.
FIG. 4 is a timing chart showing signals in the switching power source apparatus of FIG. 1 with the load 5 being in a short-circuit state. If the load 5 is short-circuited, the switching element Q10 is immediately stopped without a delay time. During a period in which the short-circuit state continues, intermittent ON/OFF operation of the switching element Q10 is carried out in a similar manner to the overload state of FIG. 3.
A starting time from when the DC power source E is applied to the switching power source apparatus to when the switching element Q10 starts ON/OFF operation to generate the output voltage Vout is determined by the starting current Ist from the constant current circuit CC1 and the capacitance of the capacitor C10. To shorten the starting time, the starting current Ist should be larger. The large starting current, however, results in increasing losses in the start circuit 1. In particular, if the auto-restart operation is achieved during an overload state or a short-circuit state by intermittently conducting ON/OFF operation of the switching element Q10, the larger starting current increases losses in the start circuit 1 and switching element Q10 to generate heat to break elements.
To solve this problem, Japanese Unexamined Patent Application Publication No. 2003-333840 discloses a switching power source apparatus having a start circuit including a constant current circuit. This constant current circuit supplies a starting current through a current limit resistor, a first npn-type transistor, and a current detection resistor to a power source terminal of a control circuit. The constant current circuit detects a terminal voltage of the current detection resistor with a second npn-type transistor and controls a current passed through a resistor to a base of the first npn-type transistor. A constant starting current supplied by the start circuit is passed through the current detection resistor. The current detection resistor is connected in parallel with a capacitor, so that a large starting current is supplied at the starting of the apparatus until the capacitor is charged. Once the capacitor is charged, the starting current is determined by the current detection resistor and second npn-type transistor. In this way, the constant current circuit differs a current value between the starting operation and a normal operation, to shorten a starting time and reduce losses in the start circuit.